KR100712501B1 - A spread spectrum clock generator with PVT invariant frequency modulation ratio - Google Patents

A spread spectrum clock generator with PVT invariant frequency modulation ratio Download PDF

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Publication number
KR100712501B1
KR100712501B1 KR20040090445A KR20040090445A KR100712501B1 KR 100712501 B1 KR100712501 B1 KR 100712501B1 KR 20040090445 A KR20040090445 A KR 20040090445A KR 20040090445 A KR20040090445 A KR 20040090445A KR 100712501 B1 KR100712501 B1 KR 100712501B1
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South Korea
Prior art keywords
frequency
modulation
spread spectrum
spectrum clock
phase
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KR20040090445A
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Korean (ko)
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KR20060041377A (en
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곽명보
김지영
김치원
김현구
박재현
서일원
신종신
장덕현
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삼성전자주식회사
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    • HELECTRICITY
    • H03BASIC ELECTRONIC CIRCUITRY
    • H03LAUTOMATIC CONTROL, STARTING, SYNCHRONISATION, OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
    • H03L7/00Automatic control of frequency or phase; Synchronisation
    • H03L7/06Automatic control of frequency or phase; Synchronisation using a reference signal applied to a frequency- or phase-locked loop
    • H03L7/16Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop
    • H03L7/18Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop using a frequency divider or counter in the loop
    • HELECTRICITY
    • H03BASIC ELECTRONIC CIRCUITRY
    • H03BGENERATION OF OSCILLATIONS, DIRECTLY OR BY FREQUENCY-CHANGING, BY CIRCUITS EMPLOYING ACTIVE ELEMENTS WHICH OPERATE IN A NON-SWITCHING MANNER; GENERATION OF NOISE BY SUCH CIRCUITS
    • H03B23/00Generation of oscillations periodically swept over a predetermined frequency range
    • HELECTRICITY
    • H03BASIC ELECTRONIC CIRCUITRY
    • H03LAUTOMATIC CONTROL, STARTING, SYNCHRONISATION, OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
    • H03L7/00Automatic control of frequency or phase; Synchronisation
    • H03L7/06Automatic control of frequency or phase; Synchronisation using a reference signal applied to a frequency- or phase-locked loop
    • H03L7/08Details of the phase-locked loop
    • H03L7/085Details of the phase-locked loop concerning mainly the frequency- or phase-detection arrangement including the filtering or amplification of its output signal
    • H03L7/089Details of the phase-locked loop concerning mainly the frequency- or phase-detection arrangement including the filtering or amplification of its output signal the phase or frequency detector generating up-down pulses
    • H03L7/0891Details of the phase-locked loop concerning mainly the frequency- or phase-detection arrangement including the filtering or amplification of its output signal the phase or frequency detector generating up-down pulses the up-down pulses controlling source and sink current generators, e.g. a charge pump

Abstract

The present invention relates to a spread spectrum clock generator that measures and corrects modulation rates on a circuit to obtain a constant modulation rate independent of PVT. Modulation ratio measurements are made by comparing the number of spread-spectrum clocks with a predetermined value over a given interval, and PVT adjustment is achieved by scaling the modulation charge pump.
PLL, SSCG, Clock Modulation

Description

Spread spectrum clock generator with PVT invariant frequency modulation ratio

1 (a) to (f) are graphs showing a change in the reference frequency of a general PLL output signal and an output signal using SSCG.

2 is a block diagram illustrating a configuration of an SSCG for controlling a modulation rate using a conventional dual loop.

3 is a block diagram showing the structure of an SSCG according to the present invention.

4 is a diagram illustrating a relationship between a frequency variation of an output signal and a comparison frequency with time.

5 is a graph showing an example of adjusting the frequency modulation ratio of the SSCG according to the present invention.

6 (a) is a graph illustrating a process of finding a target modulation value by the SSCG according to the present invention when the amount of the initial modulation charge pump is too small.

FIG. 6 (b) shows the magnitude of the control signal output from the modulation controller in the case of FIG. 6 (a).

7 (a) is a graph showing a process of finding a target modulation value by the SSCG according to the present invention when the amount of the initial modulation charge pump is too large.

FIG. 7 (b) shows the magnitude of a control signal output from the modulation controller in the case of FIG. 7 (a).

The present invention relates to a PLL, and more particularly, to a Spread Spectrum Clock Generator having a frequency modulation ratio that is not affected by PVT.

Phase Locked Loop (PLL) plays an integral role in digital systems. As technology advances, digital systems are becoming faster and more highly integrated, and PLLs are also becoming faster, which causes problems such as EMI (Electro Magnetic Interference). EMI appears when the magnitude of the energy of a high-frequency signal exceeds a predetermined reference value, which affects the surrounding electronic circuits and causes malfunctions. In particular, since semiconductor devices are sensitive to EMI, generation of EMI in semiconductor integrated circuits cannot be overlooked.

One way to reduce EMI is to use a Spread Spectrum Clock Generator. That is, in order to reduce EMI, the frequency of a reference signal having a large energy or power at a specific frequency is modulated into a frequency signal having a predetermined bandwidth and relatively less energy than the reference signal at a frequency within the bandwidth. For example, when the frequency of the reference signal is 1 MHz, it modulates to change to a signal between 0.99 MHz and 1.01 MHz during a predetermined repetition cycle.

In this way, if the reference frequency of the PLL is modulated such that the reference frequency is not fixed to one frequency but varies between predetermined frequencies, energy at a specific frequency is dispersed, resulting in a signal that does not affect EMI on neighboring electronic circuits.

Spread-spectrum clock generators (SSCGLs) are clock generators that reduce EMI by reducing power gain by slightly modulating the clock frequency of the PLL.

1 (a) to (f) are graphs showing a change in the reference frequency of the general PLL output signal and the output signal using SSCG described above.

Figure 1 (a) shows a clock having a constant frequency without using the SSCG. FIG. 1B shows a frequency spectrum with energy above a predetermined energy P 0 that generates EMI at a reference frequency (1 MHz in the example of FIG. 1). FIG. 1 (c) shows the frequency change of the PLL output signal over time, and is shown to have a constant reference frequency unchanged over time.

1 (d) shows a clock having a varying frequency using SSCG. FIG. 1 (e) shows a frequency spectrum in which a frequency is distributed around a reference frequency and spread below a predetermined energy P 0 . FIG. 1 (f) shows an example in which the frequency of the output signal varies with time, and as described above, the frequency is modulated to vary between 1.01 MHz and 0.99 MHz with respect to 1 MHz.

However, when implementing an SSCG using direct modulation, the modulation rate of the clock is dependent on the size of the modulation charge pump and VGA gain applied directly to the voltage control oscillator (VCO) control node. Will depend. Since both the size of the charge pump and the gain of the VCO vary with PVT (Process, Voltage, Temperature), the modulation rate of the SSCG also depends on the PVT. Too small a modulation rate does not yield sufficient spread spectrum effect, while a too large modulation rate can cause problems in the operation of the system.

When creating a spread-spectrum clock using direct modulation, conventionally, the amount of charge pump is adjusted manually or a dual-loop is used to correct a modulation ratio that changes with PVT change. The voltage at the VCO control node was monitored and used to adjust the modulation rate.

2 is a block diagram illustrating a configuration of an SSCG for controlling a modulation rate using a conventional dual loop.

As shown in FIG. 2, when sensing the voltage of the VCO control node using the dual loop, the V_MOD voltage should be sufficiently large because of the input sensitivity of the comparator (CMP). Thus, this voltage must be scaled before being added at the adder. However, since the modulation rate is typically in the order of a few% and the VCO control voltage generated in the master loop is in the unit of several volts, the accuracy of the voltage scaler must be precisely operated in the unit of several mV to realize the correct modulation rate. Also, because both loops require a filter, a large area is consumed to implement the filter, and the waveform generator accepts the filtered voltage (V_MVLL) as input, so the voltage ripple of the voltage (V_MVLL) The generator introduces a V_MOD voltage ripple, which acts as a random modulation, reducing the overall spectral spreading effect.

It is an object of the present invention to provide an SSCG having a constant modulation rate without changing the modulation rate according to PVT change.

Another technical problem to be achieved by the present invention is to provide an SSCG capable of maintaining a precise modulation ratio without much scaler.

Another technical problem to be solved by the present invention is to provide an SSCG which reduces the number of filters required in the PLL loop to reduce the area consumption due to the filter implementation.

In order to achieve the object of the present invention as described above, according to a feature of the present invention, the Spread Spectrum Clock Generator (SSCG) outputs a reference frequency signal obtained by dividing an input signal and dividing it by a predetermined value. A phase and frequency detector (PFD) for receiving a pre-divider, the reference frequency signal and a predetermined feedback signal, and generating a signal corresponding to a phase and frequency difference between the reference frequency signal and the feedback signal. A charge pump and filter unit which charge-pumps the filter in response to the signal output from the phase and frequency detector and then outputs a predetermined first control voltage, and an average of a predetermined period of the spread spectrum clock signal output from the voltage control oscillator Calculate a frequency and compare the calculated average frequency with a predetermined comparison frequency A modulation controller for controlling the charge pump to output a predetermined modulation voltage so as to correspond to a difference, an adder for adding the first control voltage and the modulation voltage to output a second control voltage, and a second control output from the adder A voltage controlled oscillator (VCO) for outputting a signal having a frequency corresponding to the second control voltage in response to a voltage, and a feedback signal that divides and spreads the spread spectrum clock signal output from the voltage controlled oscillator to a predetermined value And a main divider for outputting a signal, wherein the comparison frequency input to the modulation controller includes an upper comparison frequency and a lower comparison frequency, which are frequencies for which the output signal of the voltage control oscillator is a reference for a target modulation ratio.

The modulation controller obtains a maximum frequency, a minimum frequency, and an intermediate frequency from the spread spectrum clock signal output from the voltage controlled oscillator, compares a first average frequency between the minimum frequency and the intermediate frequency with the lower comparison frequency. Compare a second average frequency between the intermediate frequency at the maximum frequency with the upper comparison frequency.

In one embodiment of the present invention, the modulation control unit, the number of counting the rising edge of the spread spectrum clock signal while the spread spectrum clock signal output from the voltage controlled oscillator is changed from the intermediate frequency to the minimum frequency and the The number of rising edges of a signal having a lower comparison frequency is compared to compare the first average frequency and the lower comparison frequency, and the spread spectrum clock signal output from the voltage controlled oscillator is the maximum frequency at the intermediate frequency. The second average frequency and the upper comparison frequency are compared by comparing the number of rising edges of the spread spectrum clock signal and the number of rising edges of the signal having the upper comparison frequency while changing to.

Preferably, the modulation controller further comprises a modulation charge pump for generating the modulation voltage by pumping the charge in response to the comparison result of the counted edge number.

In one embodiment of the present invention, the modulation control unit, the lower comparison frequency for a period equal to the number of rising edge counted while the spread spectrum clock signal output from the voltage controlled oscillator is changed from the intermediate frequency to the minimum frequency When the number of rising edges counted in a signal having a current is increased, the modulation charge pump increases the modulation rate, and the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the minimum frequency. If the number of rising edges counted during the same period is less than the number of rising edges counted in the signal having the lower comparison frequency, the modulation rate is reduced by reducing the current of the modulated charge pump.

In one embodiment of the present invention, the modulation controller is further configured to control the upper comparison frequency during a period in which the number of rising edges counted while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency. When the number of rising edges counted in a signal having a voltage decreases, the current of the modulated charge pump is reduced to reduce a modulation ratio, and the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency. If the number of rising edges counted during the same period is smaller than the number of rising edges counted in the signal having the upper comparison frequency during the same period, the current of the modulation charge pump is increased to increase the modulation ratio.

In another embodiment of the present invention, the modulation controller is configured to: equal the total amount of change in phase of the spread spectrum clock signal while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the minimum frequency. While comparing the total change in phase of the signal having the lower comparison frequency to compare the first average frequency and the lower comparison frequency, wherein the spread spectrum clock signal output from the voltage controlled oscillator is the maximum frequency at the intermediate frequency. The second average frequency and the upper comparison frequency are compared by comparing the total variation of the phase of the spread spectrum clock signal with the total variation of the phase of the signal having the upper comparison frequency during the same time.

Preferably, the modulation controller further includes a modulation charge pump for generating the modulation voltage by pumping the charge in response to the comparison result of the total phase change amount.

In another embodiment of the present invention, the modulation control unit is further configured to compare the lower portion of the measured phase with the total amount of phase change while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the minimum frequency. If it is larger than the total variation of the phases measured in the signal with frequency, the current of the modulation charge pump is increased to increase the modulation ratio, and the spread spectrum clock signal output from the voltage controlled oscillator is shifted from the intermediate frequency to the minimum frequency. If the total change in phase measured during the change is less than the total change in phase measured in the signal having the lower comparison frequency during the same period, the current of the modulated charge pump is reduced to reduce the modulation rate.

In another embodiment of the present invention, the modulation controller is further configured to adjust the upper comparison frequency during the same period in which the total amount of phase change measured while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency. If it is larger than the total variation of the phases measured in the signal with, the current of the modulation charge pump is reduced to reduce the modulation ratio, and the spread spectrum clock signal output from the voltage controlled oscillator is changed from the intermediate frequency to the maximum frequency. If the total change in phase measured during the same period is less than the total change in phase measured in the signal having the upper comparison frequency during the same period, the current of the modulated charge pump is increased to increase the modulation ratio.

In order to fully understand the present invention, the advantages of the operability of the present invention, and the objects achieved by the practice of the present invention, reference should be made to the accompanying drawings which illustrate preferred embodiments of the present invention and the contents described in the accompanying drawings.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Like reference numerals in the drawings denote like elements.

The spreading spectrum clock generator (SSCG) proposed by the present invention calculates the change in modulation analogy according to the PVT change by digitally comparing the number of clock rising edges with a predetermined comparison value for a specific period, As a result, a method of controlling a current of a modulating charge pump to maintain a constant modulation ratio is used.

3 is a block diagram showing the structure of an SSCG according to the present invention.

Referring to FIG. 3, the SSCG 30 according to the present invention includes a predivider 31, a phase and frequency detector 32, a charge pump and filter unit 33, an adder 34, a voltage controlled oscillator 35, A main divider 36, a modulation control 37, and a modulation charge pump 38.

The predivider 31 divides the input signal RefClk and outputs a reference frequency signal which is divided into a predetermined value. The phase and frequency detector 32 receives a reference frequency signal output from the pre-divider 31 and a feedback signal output from the main divider 36, and a signal corresponding to the phase and frequency difference between the reference frequency signal and the feedback signal. Generates.

The charge pump and filter unit 33 may be charged pumped in response to a signal output from the phase and frequency detector 32 and then filtered to be a predetermined nominal control voltage (herein referred to as a first control voltage). Can be printed). The adder 34 sums the nominal control voltage (first control voltage) output from the charge pump and filter unit 33 and the modulation voltage output from the modulating charge pump 38 to sum the total control voltage; May be referred to as a second control voltage).

The voltage controlled oscillator (VCO) 35 outputs a signal having a frequency corresponding to the second control voltage in response to the second control voltage output from the adder 34. The main divider 36 divides the signal output from the VCO 35 to generate and output a feedback signal divided into a predetermined value.

The modulation control unit 37 calculates an average frequency of the output signal output from the VCO 35 for a predetermined period, compares the calculated average frequency with a predetermined comparison frequency, and outputs a control signal corresponding to the difference. do. The modulation charge pump 38 pumps a current in response to a control signal output from the modulation control unit 37 to output a modulation voltage.

On the other hand, the modulation control unit 37 receives a comparison frequency set as a reference frequency for the modulation ratio of the target reference frequency. The comparison frequency includes an upper comparison frequency as a reference of the average frequency of the high modulation frequency of the reference frequency and a lower comparison frequency as a reference of the average frequency of the low modulation frequency of the reference frequency.

The modulation control unit 37 obtains the maximum frequency, the minimum frequency, and the intermediate frequency from the signal output from the VCO 35, compares the first average frequency between the intermediate frequency and the lower comparison frequency at the minimum frequency, and maximum frequency. Compares the second average frequency between the intermediate frequency and the upper comparison frequency to output a corresponding control signal.                     

4 is a diagram illustrating a relationship between a frequency variation of an output signal and a comparison frequency with time.

Referring to FIG. 4, the output signal of the VCO has a frequency that varies between the maximum frequency and the minimum frequency according to a predetermined modulation ratio based on the intermediate frequency. In this case, the intermediate frequency is generally a target reference frequency, and the maximum frequency and the minimum frequency are frequencies modulated with the maximum width within a few percent of the intermediate frequency.

The upper comparison frequency and lower comparison frequency are used to measure the modulation ratio of the spread spectrum clock in the upper count region and the lower count region, respectively, as a reference for the target modulation ratio. Here, the upper count region is a period in which the spread spectrum clock signal output from the VCO modulates from the intermediate frequency to the maximum frequency, and the lower count area is a period in which the spread spectrum clock signal modulates the intermediate frequency to the minimum frequency.

The reason for separating the comparison frequency into two upper and lower comparison frequencies instead of one intermediate frequency is to control the modulation ratio of the VCO output signal. The average frequency between the minimum and maximum frequencies is the maximum frequency and the minimum frequency. This is because the magnitudes of the frequencies cannot be compared, and only the intermediate frequencies can be compared. Therefore, the magnitudes of the maximum frequency and the minimum frequency can be compared only by comparing the average frequencies of the upper count region larger than the intermediate frequency and the lower count region smaller than the intermediate frequency.

Once the final modulation ratio is set, the first average frequency of the spread spectrum clock in the upper count region should be equal to the upper comparison frequency, and the second average frequency of the spread spectrum clock in the lower count region should be equal to the lower comparison frequency.

Since the modulation method in the modulation control unit 37 is a triangular modulation method, the period of modulation can be precisely controlled. Therefore, the largest frequency deviation can be measured by the phase change during a specific time (upper count area or lower count area). This is the same as finding the area to find the height of the triangle with the base known. And if the bottom is the same, the height can be compared only by comparing the area.

The area indicated by the contrast in FIG. 4 is the total amount of change in phase, which corresponds to the area of the triangle. It is also measurable by the number of rising edges of the clock during that time.

As described above, when the final modulation ratio is set, the area formed by the upper comparison frequency and the lower comparison frequency has the same area as the triangle formed by the spread spectrum clock signal (that is, the total phase change amount), where the upper count area The first average frequency of the spread spectrum clock at is equal to the upper comparison frequency, and the second average frequency of the spread spectrum clock at the lower count region is equal to the lower comparison frequency.

That is, the modulation control unit 37 controls the lower portion of the spread spectrum clock signal output from the voltage controlled oscillator during the same time as the total change amount of the phase of the spread spectrum clock signal while the intermediate frequency changes from the intermediate frequency to the minimum frequency. Compare the first average frequency and the lower comparison frequency by comparing the total amount of change in phase of a signal having a comparison frequency, and while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency The total change amount of the phase of the spread spectrum clock signal and the total change amount of the phase of the signal having the upper comparison frequency during the same time period are compared to compare the second average frequency with the upper comparison frequency. The modulation control unit 37 then outputs a control signal corresponding to the comparison result.

Then, the modulating charge pump 38 pumps charge in response to the control signal output from the modulation controller 37 to generate the modulation voltage.

For example, a modulated charge pump 38 has the lower comparison frequency for the same period of time as the total amount of phase change measured while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the minimum frequency. If it is larger than the total amount of phase change measured in the signal, the current of the modulation charge pump 38 is increased to increase the modulation ratio, and the spread spectrum clock signal output from the voltage controlled oscillator is shifted from the intermediate frequency to the minimum frequency. If the total change in phase measured during the change is less than the total change in phase measured in the signal having the lower comparison frequency during the same period, the current of the modulated charge pump 38 is reduced to make the modulation rate small.

In addition, a modulated charge pump 38 is provided at a signal having the upper comparison frequency for the same period in which the total amount of phase change measured while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency. When larger than the total amount of phase change measured, the current of the modulation charge pump 38 is reduced to reduce the modulation rate, while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency. If the total amount of change in the measured phase is less than the total amount of change in the phase measured in the signal having the upper comparison frequency during the same period, the current of the modulation charge pump 38 is increased to increase the modulation ratio.

On the other hand, the total amount of change in phase can be calculated in digital form by counting the rising edge of the clock. For example, if the number of rising edge counts of the clock is 30, it can be estimated that the total amount of change in phase is approximately 30 x 360 degrees. Therefore, the average of the frequencies may be calculated through the count of the rising edges of the clock.

That is, in another embodiment of the present invention, the modulation control unit 37 may determine the number of rising edges of the spread spectrum clock signal while the spread spectrum clock signal output from the voltage controlled oscillator is changed from the intermediate frequency to the minimum frequency. The number of rising edges of the signal having the lower comparison frequency is compared to compare the first average frequency and the lower comparison frequency, and the spread spectrum clock signal output from the voltage controlled oscillator is converted from the intermediate frequency to the maximum frequency. During the change, the second average frequency is compared with the upper comparison frequency by comparing the number of rising edges of the spread spectrum clock signal with the number of rising edges of the signal having the upper comparison frequency.                     

Then, the modulated charge pump 38 pumps electric charges so as to correspond to the comparison result of the counted edge numbers, and outputs a modulated voltage.

For example, modulated charge pump 38 may have a signal having the lower comparison frequency for a period in which the number of rising edges counted is equal while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the minimum frequency. If more than the number of rising edges counted at, increase the modulation rate by increasing the current of the modulated charge pump 38 and counting while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the minimum frequency. When the number of rising edges is smaller than the number of rising edges counted in the signal having the lower comparison frequency for the same period, the current of the modulating charge pump 38 is reduced to make the modulation ratio small.

In addition, the modulating charge pump 38 counts at a signal having the upper comparison frequency for the same number of rising edges counted while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency. If it is larger than the number of rising edges, the current of the modulation charge pump 38 is reduced to make the modulation rate small, and the rising edge counted while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency. If the number of times is smaller than the number of rising edges counted in the signal having the upper comparison frequency for the same period, the current of the modulating charge pump 38 is increased to increase the modulation rate.                     

On the other hand, the modulation voltage output from the modulating charge pump 38 is summed in the summation section 34 with the first control voltage output from the charge pump and filter section 33. The adder 34 may be configured as a capacitor. The adder 34 may add a first control voltage and a modulation voltage to output a third control voltage that modulates as much as the modulation voltage based on the first voltage.

The VCO 35 generates a spread spectrum clock signal that modulates the reference frequency, that is, the intermediate frequency of FIG. 4, in response to the third control voltage.

In other words, the spread spectrum clock generator according to the present invention can digitally compare the comparison frequency and the frequency of the feedback spread spectrum clock, so that it is not influenced by PVT, and there is no need for a separate scaler or additional filter in the PLL. The modulation ratio can be maintained and the area of the circuit can be reduced.

5 is a graph showing an example of adjusting the frequency modulation ratio of the SSCG according to the present invention.

5A illustrates an example in which the maximum frequency of the spread spectrum clock SSC output from the VCO is smaller than the target maximum frequency and thus the output clock signal is not spread as much as desired. In this case, the energy of the clock signal may exceed the predetermined range and affect the surrounding circuits of the EMI.

When the actual maximum SSC frequency is smaller than the target maximum frequency as shown in FIG. 5A, the upper comparison frequency is larger than the second average frequency, which is the average frequency while the spread spectrum clock (SSC) signal is changed from the intermediate frequency to the maximum frequency. As a result, the number of clocks of the signal having the upper comparison frequency during the same time is greater than the number of clocks of the signal having the second average frequency. Thus, the modulation controller 37 determines that the number of rising edges counted while the spread spectrum clock (SSC) signal changes from the intermediate frequency to the maximum frequency is equal to the number of rising edges counted in the signal having the upper comparison frequency for the same period. It is judged to be smaller than the number. In this case, the modulation control unit 37 increases the current of the modulation charge pump 38 to increase the modulation ratio to bring the maximum frequency of the SSC closer to the target maximum frequency.

5B illustrates an example in which the frequency modulation deviation of the output clock signal is greater because the maximum frequency of the spread spectrum clock SSC output from the VCO is greater than the target maximum frequency. In this case, the frequency modulation width of the clock signal is large, which may cause malfunction of the electronic circuit.

When the actual maximum SSC frequency is larger than the target maximum frequency as shown in FIG. 5B, the upper comparison frequency is smaller than the second average frequency, which is the average frequency while the spread spectrum clock (SSC) signal is changed from the intermediate frequency to the maximum frequency. As a result, the number of clocks of the signal having the upper comparison frequency during the same time is less than the clock number of the signal having the second average frequency. Thus, the modulation controller 37 determines that the number of rising edges counted while the spread spectrum clock (SSC) signal changes from the intermediate frequency to the maximum frequency is equal to the number of rising edges counted in the signal having the upper comparison frequency for the same period. Judging from the number. In this case, the modulation control unit 37 reduces the current of the modulation charge pump 38 to reduce the modulation ratio to bring the maximum frequency of the SSC closer to the target maximum frequency.                     

5C illustrates an example in which the output clock signal is not spread as much as desired because the minimum frequency of the spread spectrum clock SSC output from the VCO is higher than the target minimum frequency. In this case as well, as shown in FIG. 5A, the energy of the clock signal may exceed the predetermined range and affect the surrounding circuits of the EMI.

When the actual minimum SSC frequency is higher than the target minimum frequency as shown in FIG. 5C, the lower comparison frequency is lower than the first average frequency, which is the average frequency while the spread spectrum clock (SSC) signal is changed from the intermediate frequency to the minimum frequency. As a result, the number of clocks of the signal having the lower comparison frequency during the same time is less than the number of clocks of the signal having the first average frequency. Thus, the modulation control 37 determines that the number of rising edges counted while the spread spectrum clock (SSC) signal changes from the intermediate frequency to the minimum frequency is equal to the number of rising edges counted in the signal having the lower comparison frequency for the same period. Judging by the number. In this case, the modulation control unit 37 increases the current of the modulation charge pump 38 to increase the modulation ratio to bring the minimum frequency of the SSC closer to the target minimum frequency.

FIG. 5D illustrates an example in which the frequency modulation deviation of the output clock signal is large because the minimum frequency of the spread spectrum clock SSC output from the VCO is lower than the target minimum frequency. In this case, too, as in FIG. 5B, the frequency modulation width of the clock signal is large, which may cause malfunction of the electronic circuit.

When the actual minimum SSC frequency is lower than the target minimum frequency as shown in FIG. 5 (d), the lower comparison frequency is higher than the first average frequency which is the average frequency while the spread spectrum clock (SSC) signal is changed from the intermediate frequency to the minimum frequency. . As a result, the number of clocks of the signal having the lower comparison frequency during the same time is greater than the number of clocks of the signal having the first average frequency. Thus, the modulation control 37 determines that the number of rising edges counted while the spread spectrum clock (SSC) signal changes from the intermediate frequency to the minimum frequency is equal to the number of rising edges counted in the signal having the lower comparison frequency for the same period. I think it is less than the number. In this case, the modulation control unit 37 reduces the current of the modulation charge pump 38 to decrease the modulation ratio to bring the minimum frequency of the SSC to the target minimum frequency.

6 (a) is a graph illustrating a process of finding a target modulation value by the SSCG according to the present invention when the amount of the initial modulation charge pump is too small.

FIG. 6 (b) shows the magnitude of the control signal output from the modulation controller in the case of FIG. 6 (a).

6 (a) and 6 (b), if the initial modulation amount is too small, the frequency modulation width of the spread spectrum clock becomes small, and in the upper count region, the total count of the spread spectrum clock is smaller than the count of the target clock. In response, the modulation controller increases the amount of the charge pump by increasing the size of the control signal.

7 (a) is a graph showing a process of finding a target modulation value by the SSCG according to the present invention when the amount of the initial modulation charge pump is too large.

FIG. 7 (b) shows the magnitude of a control signal output from the modulation controller in the case of FIG. 7 (a).                     

Referring to FIGS. 7A and 7B, when the initial modulation amount is too large, the frequency modulation width of the spread spectrum clock becomes large, and in the upper count region, it is determined that the total count number of the spread spectrum clock is larger than the count number of the target clock. In addition, the modulation controller reduces the amount of the charge pump by reducing the size of the control signal.

As a result, a stable spread spectrum clock signal can be generated by finding the target modulation ratio through digital control without being affected by PVT.

Although the present invention has been described with reference to one embodiment shown in the drawings, this is merely exemplary, and those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. . Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

According to the spread spectrum clock generator according to the present invention, it is possible to maintain a constant modulation rate without changing the modulation rate according to the PVT change, maintain a precise modulation rate without much scale, and reduce the area consumption due to the circuit configuration.

Claims (18)

  1. In Spread Spectrum Clock Generator (SSCG),
    A pre-divider for dividing an input signal and outputting a reference frequency signal divided by a predetermined value;
    A phase and frequency detector (PFD) for receiving the reference frequency signal and a predetermined feedback signal and generating a signal corresponding to a phase and frequency difference between the reference frequency signal and the feedback signal;
    A charge pump and filter unit configured to output a predetermined first control voltage by performing charge pumping and filtering in response to the signal output from the phase and frequency detector;
    A modulation that calculates an average frequency of a predetermined period of the spread spectrum clock signal output from the voltage controlled oscillator, compares the calculated average frequency with a predetermined comparison frequency, and controls a charge pump to correspond to the difference to output a predetermined modulation voltage. Control unit;
    An adder configured to add the first control voltage and the modulation voltage to output a second control voltage;
    A voltage controlled oscillator (VCO) for outputting a signal having a frequency corresponding to the second control voltage in response to a second control voltage output from the adder; And
    A main divider which divides the spread spectrum clock signal output from the voltage controlled oscillator and outputs a feedback signal divided by a predetermined value,
    The comparison frequency input to the modulation control unit is a spread spectrum clock generator, characterized in that the output frequency of the voltage control oscillator includes a frequency and the upper comparison frequency as a reference for the target modulation ratio.
  2. The method of claim 1,
    The modulation controller obtains a maximum frequency, a minimum frequency, and an intermediate frequency from the spread spectrum clock signal output from the voltage controlled oscillator, compares a first average frequency between the minimum frequency and the intermediate frequency with the lower comparison frequency. And comparing a second average frequency between said intermediate frequency at said maximum frequency with said upper comparison frequency.
  3. The method of claim 2,
    The modulation control unit,
    The number of rising edges of the spread spectrum clock signal and the number of rising edges of the signal having the lower comparison frequency while the spread spectrum clock signal output from the voltage controlled oscillator is changed from the intermediate frequency to the minimum frequency. Compares the first average frequency with the lower comparison frequency,
    The number of rising edges of the spread spectrum clock signal and the number of rising edges of the signal having the upper comparison frequency while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency. Spread spectrum clock generator, characterized in that comparing the second average frequency and the upper comparison frequency.
  4. The method of claim 3, wherein
    And a modulation control pump to generate the modulation voltage by pumping charge in response to the comparison result of the counted edge numbers.
  5. The method of claim 4, wherein
    The modulation control unit may include: a rising edge counted from a signal having the lower comparison frequency for a period equal to the number of rising edges counted while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the minimum frequency If more than, the current of the modulating charge pump is increased to increase the modulation ratio,
    The number of rising edges counted while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the minimum frequency is less than the number of rising edges counted in the signal having the lower comparison frequency for the same period Spread spectrum clock generator, characterized in that to reduce the modulation ratio by reducing the current of the modulated charge pump.
  6. The method of claim 4, wherein
    The modulation control unit may include: a rising edge counted from a signal having the upper comparison frequency for a period equal to the number of rising edges counted while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency If more than, the current of the modulating charge pump is reduced to make the modulation rate small.
    The number of rising edges counted while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency is less than the number of rising edges counted in the signal having the upper comparison frequency for the same period And increasing the current of the modulated charge pump to increase a modulation ratio.
  7. The method of claim 2,
    The modulation control unit,
    The total amount of change in phase of the spread spectrum clock signal while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the minimum frequency and the total amount of change in phase of the signal having the lower comparison frequency during the same time. Compares the first average frequency with the lower comparison frequency,
    The total amount of change in phase of the spread spectrum clock signal while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency and the total of the phase of the signal having the upper comparison frequency during the same time And comparing the variation amount to compare the second average frequency and the upper comparison frequency.
  8. The method of claim 7, wherein
    And a modulation charge pump to generate the modulation voltage by pumping charge in response to the comparison result of the total amount of phase change.
  9. The method of claim 8,
    The modulation controller is further configured to adjust the phase of the phase measured in the signal having the lower comparison frequency during the same period in which the total change in phase measured while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the minimum frequency. If it is larger than the total change amount, the current of the modulation charge pump is increased to increase the modulation ratio,
    When the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the minimum frequency, the total change in phase measured is less than the total change in phase measured in the signal having the lower comparison frequency during the same period. Spread spectrum clock generator, characterized in that to reduce the modulation ratio by reducing the current of the modulated charge pump.
  10. The method of claim 8,
    The modulation controller is further configured to adjust the phase of the phase measured in the signal having the upper comparison frequency during the same period in which the total change in phase measured while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency. If larger than the total change amount, the modulation charge pump current is reduced to reduce the modulation rate,
    When the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency, the total change in phase measured is less than the total change in phase measured in the signal having the upper comparison frequency for the same period. And increasing the current of the modulated charge pump to increase a modulation ratio.
  11. The method of claim 1,
    And said adder comprises a capacitor.
  12. A pre-divider for dividing an input signal and outputting a reference frequency signal divided by a predetermined value;
    A phase and frequency detector (PFD) for receiving the reference frequency signal and a predetermined feedback signal and generating a signal corresponding to a phase and frequency difference between the reference frequency signal and the feedback signal;
    A charge pump and filter unit configured to output a predetermined first control voltage by performing charge pumping and filtering in response to the signal output from the phase and frequency detector;
    Compute the total phase change amount of the spread spectrum clock signal output from the voltage controlled oscillator for a predetermined period, compare the calculated total phase change amount with the total phase change amount of the predetermined comparison frequency, and control the charge pump to correspond to the difference. A modulation control unit for outputting a modulation voltage;
    An adder configured to add the first control voltage and the modulation voltage to output a second control voltage;
    A voltage controlled oscillator (VCO) for outputting a signal having a frequency corresponding to the second control voltage in response to a second control voltage output from the adder; And
    A main divider which divides the spread spectrum clock signal output from the voltage controlled oscillator and outputs a feedback signal divided by a predetermined value,
    The comparison frequency input to the modulation controller includes a phase locked loop (PLL), which includes an upper comparison frequency and a lower comparison frequency, which are frequencies for which the output signal of the voltage controlled oscillator is a reference for a target modulation ratio. .
  13. The method of claim 12,
    The modulation control unit further comprises a modulation charge pump for generating the modulation voltage by controlling the current of the charge pump to correspond to the comparison result of the total phase change amount.
  14. The method of claim 13,
    The modulation control unit,
    The total amount of change in phase of the spread spectrum clock signal while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the minimum frequency and the total amount of change in phase of the signal having the lower comparison frequency during the same time. Compare and
    The total amount of change in phase of the spread spectrum clock signal while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency and the total amount of change in phase of the signal having the upper comparison frequency during the same time. Comparing the PLL.
  15. The method of claim 14,
    The modulation controller is further configured to adjust the phase of the phase measured in the signal having the lower comparison frequency during the same period in which the total change in phase measured while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the minimum frequency. If it is larger than the total change amount, the current of the modulation charge pump is increased to increase the modulation ratio,
    When the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the minimum frequency, the total change in phase measured is less than the total change in phase measured in the signal having the lower comparison frequency during the same period. And reducing the current of the modulating charge pump to reduce the modulation ratio.
  16. The method of claim 14,
    The modulation controller is further configured to adjust the phase of the phase measured in the signal having the upper comparison frequency during the same period in which the total change in phase measured while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency. If larger than the total change amount, the modulation charge pump current is reduced to reduce the modulation rate,
    When the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency, the total change in phase measured is less than the total change in phase measured in the signal having the upper comparison frequency for the same period. And increasing the current of the modulating charge pump to increase the modulation ratio.
  17. The method of claim 14,
    The modulation control unit,
    The number of rising edges of the spread spectrum clock signal and the number of rising edges of the signal having the lower comparison frequency while the spread spectrum clock signal output from the voltage controlled oscillator is changed from the intermediate frequency to the minimum frequency. To compare the total amount of change in the phase,
    The number of rising edges of the spread spectrum clock signal and the number of rising edges of the signal having the upper comparison frequency while the spread spectrum clock signal output from the voltage controlled oscillator changes from the intermediate frequency to the maximum frequency. PLL by comparing the total amount of change of the phase by comparing the.
  18. The method of claim 12,
    The summation unit PLL, characterized in that consisting of a capacitor.
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US11/205,014 US7558311B2 (en) 2004-11-08 2005-08-17 Spread spectrum clock generator and method for generating a spread spectrum clock signal
TW94136499A TWI310636B (en) 2004-11-08 2005-10-19 Spread spectrum clock generator and method for generating a spread spectrum clock signal
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